1. Technical Field
The present disclosure relates to a pulse width modulation (PWM) modulator and a class-D amplifier having the same, and more particularly, to a pulse width modulation (PWM) modulator that can reduce electromagnetic interference (EMI), and a class-D amplifier having the same.
2. Discussion of the Related Art
Typically, amplifiers are classified into analog amplifiers and digital amplifiers. The analog amplifiers include class-A amplifiers, class-B amplifiers, and class-AB amplifiers while the digital amplifiers include class-D amplifiers. The efficiency of the analog amplifiers, for example, the class-A amplifier, the class-B amplifier, and the class-AB amplifier, is not good, and the power loss of the analog amplifier is very high. Thus, a lot of heat is generated in the analog amplifier, so that the temperature of the analog amplifier increases.
In the analog amplifier, since a large heat sink or cooling fan is needed to forcibly dissipate the heat generated from the analog amplifier, the size of the analog amplifier is increased. Also, when the temperature of an audio apparatus including the analog amplifier increases, the analog amplifier may be damaged. Furthermore, in the analog amplifier using a vacuum tube or a transistor, distortion of the signal may be unavoidable and is caused by thermal noise due to thermal motion of electrons and the instable linearity of an amplification device, for example, a vacuum tube or a transistor.
In contrast, the class-D amplifier has a high efficiency and is small and light, thus providing portability. Also, since the power consumed that results in heat in the class-D amplifier itself is small, the class-D amplifier is widely used.
FIG. 1 is a functional block diagram of a general class-D amplifier. Referring to FIG. 1, a class-D amplifier 5 includes an input gain stage 10, a PWM modulator 20, and two power stages 30 and 40.
An audio signal input to the input gain stage 10 is an analog signal and may be an input signal of an audio apparatus that is commonly used. The input gain stage 10 receives and amplifies the analog signal. The PWM modulator 20 includes a circuit (not shown) in the form of a comparator, and compares the input analog signal with a predetermined reference and generates a PWM signal according to the result of comparison.
For example, when the level of the input analog signal is greater than that of the reference, the PWM modulator 20 outputs a high level or “1”. When the level of the input analog signal is less than that of the reference, the PWM modulator 20 outputs a low level or “0”. That is, the PWM modulator 20 generates a PWM signal based on the input analog signal and the reference.
Each of the power stages 30 and 40 receives the PWM signal output from the PWM modulator 20, buffers, and outputs the buffered signal to an inductive load, for example, a speaker. The class-D amplifier 5 is a full bridge type amplifier having the two power stages 30 and 40. The full bridge type amplifier 5 may generate a large output power. Meanwhile, a class-D amplifier using one only power stage is a half bridge type amplifier.
FIG. 2 is a block diagram of a conventional PWM modulator having a ramp generator. As shown in FIG. 2, in a PWM modulator 20 having a ramp generator 23, which is disclosed in U.S. Pat. No. 6,262,632, comparators 40 and 42 respectively receive ramp signals 36 and 38 generated from the ramp generator 23 and output signals of integrators 24 and 26 and respectively generate PWM signals based on the integrated signals and the respective ramp signals 36 and 38.
Since the PWM modulator 20 requires the ramp generator 23 shown in FIG. 2, the structure of the PWM modulator 20 is complicated and difficult to realize. Also, since switching noise of each of the output bridges 28 and 30 can be included in the output signals of the integrators 24 and 26, distortion may be generated in the output signals of the integrators 24 and 26.
FIG. 3 is a block diagram showing a conventional self-oscillation PWM modulator. As shown in FIG. 3, the self-oscillation PWM modulator, which is disclosed in U.S. Pat. No. 6,362,702, does not include the ramp generator unlike the PWM modulator shown in FIG. 2, although it uses two integrators 11 and 12. Since the switching noise at an output terminal 6 of the self-oscillation PWM modulator can be introduced to the integrator 12, an output signal Vb of the integrator 12 can be distorted.
FIG. 4 is a circuit diagram of a conventional power stage of the class-D amplifier. Referring to FIGS. 1 and 4, the power stage 30 of FIG. 4 is a half-bridge type constituting anyone of the power stages 30 and 40 of FIG. 1.
The power stage 30 receives a PMOS input signal PMOS_IN and an NMOS input signal NMOS_IN generated in response to a signal output from the PWM modulator 20. Since a plurality of transistors 31-37 are operated in response to the received input signals PMOS_IN and NMOS_IN, the power stage 30 outputs a high power output signal to a speaker. The conventional power stage 30 includes a power source 39, a plurality of inverters IN1 and IN2, a pull-up transistor 36, and a pull-down transistor 37.
The power stage 30 obtains a high output power by increasing the output current per unit time using the pull-up transistor 36 and the pull-down transistor 37, which have a high ratio of length to width of a channel, hereinafter referred to as the “W/L”. Due to this rapid change in the output current, EMI is considerably increased in the power stage 30.
Also, since the switching peak voltage caused by the switching operation of the power stage 30 is high, the power stage 30 generates a lot of harmonic components. The harmonic wave components cause EMI in a surrounding circuit, so that a malfunction of the surrounding circuit may occur.